As processes for producing an optical isomer of compound (II) ##STR2## wherein X, Y and Z, which may be the same or different, each represents a hydrogen atom or a halogen atom and R represents a lower alkyl group having 1 to 6 carbon atoms, a process comprising converting compound (II) into an optically active proline derivative and isolating said product is known as described, for example, in EP-A-206,283. However, this procedure is disadvantageous in that the resolution reagent proline is expensive and is difficult to use again. Another known process involves the use of an asymmetric hydrolytic enzyme as described, for example, in JP-A-62-87577 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") and EP-A-206,283.